This invention relates to a method of fabricating optical waveguiding devices, such as those used in optical communication systems.
Much recent work has been directed toward the fabrication of optical circuit devices for use in future optical communication systems and other optical apparatus. See, for example, Volume QE-8 of the IEEE Journal of Quantum Electronics, page 199 (1972). The basic building blocks of such devices are dielectric waveguides, which consist of an optically transparent waveguiding region surrounded by media of lower indices of refraction. One can distinguish in the art between two fundamental classes of approaches to the fabrication of these waveguides.
In the first class, optical waveguides are typically formed by selectively applying a layer of a dielectric material onto a substrate of lower refractive index. In this class are found techniques such as that described in Volume 48 of the Bell System Technical Journal, page 3445 (1969), wherein high optical quality waveguides are fabricated by sputtering of optical glasses through a mask onto lower index glass substrates. Epitaxial growth techniques, such as that described in Volume 21 of Applied Physics Letters, page 358 (1972), have been found attractive for the growth of high quality single crystalline guiding layers on lower index crystalline substrates. Each of these approaches is relatively simple and well suited for the inexpensive mass production of the desired devices. However, the devices fabricated according to these techniques are characterized by sharp, step-like transitions of refractive index between the different dielectric materials. This fact, combined with the presence of any roughness or irregularities in the material interfaces, can give rise to undesirably high scattering losses for guided optical waves.
In the second class of fundamental approaches, waveguides are typically formed by selectively altering the index of refraction of a bulk transparent sample. Here one finds techniques such as the ion bombardment technique described in Volume 11 of Applied Optics, page 1313 (1972), wherein selected regions of increased refractive index are provided by selectively generating molecular disorder in a bulk sample. Also in this class are techniques in which selected regions of changed index of refraction are induced either by diffusing an index changing diffusant into a transparent sample, as described in Volume 21 of Applied Physics Letters, page 325 (1972), or by effusing (i.e., outdiffusing) selected chemical constituents from bulk samples to effect a change in index, as described in the article by two of us (Carruthers and Kaminow), in Volume 22 of Applied Physics Letters, page 326 (1973). In each of these approaches, the index transitions in the guides are smooth and gradual, rather than sharp. This fact eliminates, for the most part, the imperfect dielectric interfaces responsible for high scattering losses in waveguides of the first class, and thus makes the waveguides of the second class generally more desirable. Nevertheless, each of these approaches has been found to have inherent weaknesses. For example, in the ion bombardment technique, a trade-off is typically required between large index changes on the one hand, and low bulk scattering losses and high resolution on the other. The diffusion techniques have to date been shown to be useful only with a limited class of diffusant and host materials. Finally, the guides produced by the outdiffusion technique have been hampered to date by relatively low index changes spread over relatively large sample thicknesses.